What "Size" are your tires? Part 1 of 3

 

What size tire is it?          This is Part one of a three part series:

That seems to be an early response whenever you ask a question about tires. Whether you ask about price, load capacity, or inflation, the first reply may be “What size is it?”  The answer you give should not be “It’s a 22.5”  or  “it’s a 225-R-15”. These are just partial answers and indicate to many that you don’t really understand all you should about your tires.

The reason you need to provide the complete tire description information is simply because there are so many possible replies and starting to narrow down the possibilities by properly identifying the tire is just the first step in learning the details so the person that is offering the help can provide the correct answer you seek and not just a wild guess. Knowing your complete tire description can be confusing and sometimes it seems as though we tire engineers and government agencies have conspired to make things difficult. What you need to remember is that there are three basic features that must be established first. Tire type, tire physical dimensions and tire strength.

This article covers a number of different type tires. Please do not skip over any part as the knowledge provided here builds on previous covered information.

Let’s start off with TYPE. This is usually a function of application. For most tires, there is a letter code as the first part of a “Complete” identification of a tire. For most tires sold in the US the code for tire type is either a “P” for Passenger, “LT” for Light Truck, “ST” for Special Trailer, or no letter for commercial or heavy duty. Tire engineers sometimes call these commercial sizes “TBR” type, which is short for Truck Bus Radial. If you are reading this article, most likely you have a Recreational Vehicle or RV of some sort so the use and application of tires on the various type of RV will be our focus. There are of course many other type tires. OTR for Off the Road or AG for agricultural or AT for All-Terrain or M for Motorcycle and others, but we will not cover those and will focus on the type, size and strength tires used in various RV application.

First, we will cover “P” type tires. Most of us own or have owned some form of passenger car that came with P-type tires. Older and smaller trailers may also come with "P" type tires mounted by the manufacturer. When our car required replacement tires, we seldom needed to think much about the proper size nomenclature, as it was the responsibility of the tire dealer to confirm the appropriate type and size tire that was needed.  Our car tires would probably be identified as a P195/75R15 94S or similar combination of letters and numbers. The P as you now know indicates Passenger car application. 195 is the width in mm.  Not the tread width but the maximum width. 75 is a ratio of the tire height from the wheel to the tread as a percentage of the tire width. “R” stands for radial. Since there are very few non-radial “D” or “Diagonal” construction tires, we don’t need to go down the road of old tire construction. The "15" is the wheel size. Finally, there is the “Service Description” which is a combination of “Load Index” number, 94 in our example and finally the “S” is the Speed Symbol. In the US, the speed symbol is really just an indication of the level of handling capability or steering response with increased handling potential as we move from Q to R, then S followed by T, U, H, V, W, Y and finally Z. Unlike Europe where you are required to replace tires with the same speed symbol, we in the US have the option of changing the rating but should expect the steering response to get slower if we go to a lower symbol. The final bit of information concerning the use of P type tires in RV application, the load capacity of a P type tire must be reduced by dividing by 1.10 when using a P type on a truck, trailer or “Multi-Purpose Vehicle” (SUV or Station Wagon) according to tire industry design standards.

We will continue with ST a LT and other type tires in Part 2 and 3.